The present invention relates to the inspection of pattern for detecting defects (short-circuits, line breakages, etc.) and foreign particle on a pattern under test, and particularly to a method and apparatus of pattern inspection for detecting defects and foreign particle on a pattern of semiconductor wafer, liquid crystal display panel, photomask, etc. In the following explanation, the term “defect” is used to signify also foreign particle inclusively.
There is a conventional pattern defect inspection apparatus which is designed to image a pattern under test with an imaging device such as a line sensor, while moving the pattern, and compare the image signal in terms of tone levels with the image signal which has been delayed by a prescribed time length, thereby recognizing the inequality to be the presence of a defect, as described in Japanese Patent Laid-Open No. H7 (1995)-318326.
There is known a conventional technique pertinent to the pattern defect inspection, as disclosed in Japanese Patent Laid-Open No. H8(1996)-320294. The technique of this patent publication deals with the high-accuracy inspection of microscopic defects of a pattern formed on a semiconductor wafer or the like where each chip includes high pattern density areas such as memory mats and low pattern density areas such as peripheral circuits. Specifically, the pattern is imaged and a resulting image signal is D/A converted, and a resulting digital image signal is rendered the gradating conversion such that the high pattern density areas and low pattern density areas have a certain relation in terms of brightness or contrast based on the frequency distribution of brightness of the image signal. The image signal resulting from gradating conversion is compared with a comparative image signal resulting from gradating variation in a state of position matching thereby to detect accurately a microscopic defect.
There is known a conventional technique for inspecting a pattern of photo-mask, as disclosed in Japanese Patent Laid-Open No. H10(1998)-78668. The technique of this patent publication uses a light source of UV laser such as excimer laser, with the coherency of laser being diminished by a revolving diffusion plate which is placed to cut in the light path, to illuminate the photo-mask uniformly thereby to image the pattern, and calculates the characteristic value from a resulting image data to assess the quality of photo-mask.
There are excimer laser exposure apparatus as disclosed, for example, in Japanese Patent Laid-Open Nos. S59(1984)-226317 and Sho. 62(1987)-231924.
In the recent LSI manufacturing, circuit patterns formed on wafers have their width decreased to become 0.25 μm or less to meet the demand of high-density integration, and this dimension is the limit of resolution of the imagery optical system. Therefore, the application of high NA based design of imagery optical system and super-high resolution technique are in progress.
However, the high NA based design is already at a physical limit, and the breakthrough approach is to shorten the wavelength of imaging light into the ranges of UV (ultraviolet) wavelengths of 300-380 nm, DUV (deep ultraviolet) wavelengths of 190-300 nm, VUV (vacuum ultraviolet) wavelengths of 100-190 nm, and EUV (extreme ultraviolet) wavelengths of the order of 10 nm.
Due to the requirement of high-speed inspection, the scheme of subject scanning with a spot-focused laser beam cannot be adopted. Illumination of subject with a wide full-field laser light creates speckles, which cause overshooting and undershooting called “wringing” at edge of circuit pattern, resulting in a degraded image quality.